The present invention relates to screws.
In particular, the present invention relates to the threading of screws.
In the manufacture of screws, bolts, and any threaded fasteners, it is most common to employ thread-rolling dies in order to form threads on the screw shank.
In machines which are manufactured for this purpose, a headed blank is fed into a working position between a pair of opposed thread-rolling dies. These dies are simultaneously moved in opposite directions so that the screw shank which is to be threaded is rolled therebetween, and the dies are pressed toward each other, so that the thread which is on the dies is impressed into the blank screw shank. As a result the screw emerges from the dies with whatever thread was on the die faces. With structures of this type the shape of the thread may have any desired conformation, and the same is true of the particular helix angle of the thread.
Thread-rolling dies are conventionally manufactured by crush grinding in the case of a screw which has a straight cylindrical shank along its entire length. Thus, in the case of a machine screw it is possible to use such thread-rolling dies. However, the thread-rolling dies may also be manufactured by milling operations while the die blocks are in a soft condition. After the milling operations are completed the dies are hardened.
Dies of the above type, which have flat die faces, are simple dies and are not expensive to manufacture. However, when manufacturing screws which taper at the free end regions of the shanks so as to have a lead-in tip portion, the thread-rolling dies must have a configuration capable of forming the thread on the tapered tip region of the screw shank as well as on the non-tapered cylindrical shank portion which is of constant diameter all the way from the tapered tip region up to the head. Dies which can roll threads on such screw shanks obviously cannot be made by crush grinding or simple milling operations. Instead it is essential to manufacture such dies with a tool having a single cutting point, with the successive ridges and valleys being formed in the die face one at a time. There is machinery for carrying out operations of this type where, by means of a cam, the tool is caused to rise at the required angle at the areas where the die face is inclined so as to form the thread on the tapered free end region of the screw shank.
Equipment of this latter type is extremely expensive, and the operations in connection with cutting of the die faces of the thread-rolling dies is extremely tedious and extremely expensive as compared with the simple milling of a die which has a simple flat surface into which the thread-rolling ridges and valleys are milled, in a manner well-known in the manufacture of dies for thread-rolling simple machine screws which have non-tapered shanks of constant diameter throughout their entire length.
In recent years there has been the development of screws which are self-tapping and which form a thread not by cutting but by a swaging action. These results have been brought about by providing screw threads of lobular configuration in that the crests of the threads have lobular projections formed thereon according to a given pattern of distribution along the thread. The lobes may be distributed in a triangular pattern so that each thread convolution has three lobes, or more or less lobes may be provided. The lobes may be arranged axially along the screw thread, along lines parallel to the axis of the screw, or the lobes may be arranged along spiral lines having any desired inclination with respect to the helix along which the thread extends.
At the present time, screws of this latter type, where lobes are oriented along straight or spiral lines, are manufactured with extremely involved and costly methods and apparatus, requiring, for example, the use of essentially predrawn triangular wire, with trilobular guides, cut-off blades, and cold-heading dies. All of the tools of this latter type are quite expensive when they require conformations of this latter type in order to achieve a trilobular configuration for the screw thread.